Nested classes deserialized from flat XML

One option is to use XmlAnyElementAttribute as below:

public class Foo
{
    public string Name { get; set; }
    public Bar Bar { get; set; }

    [XmlAnyElementAttribute]
    public XmlElement[] BarElements
    {
        get { return null; }
        set
        {
            Bar = new Bar();
            var barType = Bar.GetType();
            foreach (var prop in value)
                barType.GetProperty(prop.Name.Substring(4)).SetValue(Bar, prop.InnerText);
        }
    }
}

public class Bar
{
    public string Data { get; set; }
    public string MoreData { get; set; }
}

When XmlSerializer does not recognize an element it adds it to property of type XmlElement[] marked by XmlAnyElementAttribute. That is where you can process Bar properties. I used reflection there to show the idea.

Another option is to deserialize twice and connect Bar with Foo:

public class Foo
{
    public string Name { get; set; }
    public Bar Bar { get; set; }
}

[XmlRoot("Foo")]
public class Bar
{
    [XmlElement("Bar_Data")]
    public string Data { get; set; }
    [XmlElement("Bar_MoreData")]
    public string MoreData { get; set; }
}

var foo = (Foo) new XmlSerializer(typeof (Foo)).Deserialize(...);
var bar = (Bar) new XmlSerializer(typeof (Bar)).Deserialize(...);
foo.Bar = bar

Yet another option with no intrusion to classes being deserialized:

public class Foo
{
    public string Name { get; set; }
    public Bar Bar { get; set; }
}

public class Bar
{
    public string Data { get; set; }
    public string MoreData { get; set; }
}

var fooSerializer = new XmlSerializer(typeof (Foo));
fooSerializer.UnknownElement += (sender, e) =>
{
    var foo = (Foo) e.ObjectBeingDeserialized;
    if(foo.Bar == null)
        foo.Bar = new Bar();
    var propName = e.Element.Name.Substring(4);
    typeof(Bar).GetProperty(propName).SetValue(foo.Bar, e.Element.InnerText);
};

var fooInstance = fooSerializer.Deserialize(...);

and if double deserialization or reflection is problematic performance wise, then you could create a surrogate proxy class:

    [XmlRoot("Foo")]
    public class FooSurrogate
    {
        public string Name { get; set; }
        public string Bar_Data { get; set; }
        public string Bar_MoreData { get; set; }

        public Foo ToFoo()
        {
            return new Foo
            {
                Name = Name,
                Bar = new Bar
                {
                    Data = Bar_Data,
                    MoreData = Bar_MoreData
                }
            };
        }
    }

    var seializer = new XmlSerializer(typeof (FooSurrogate));
    var foo = ((FooSurrogate) seializer.Deserialize(...)).ToFoo();